The invention relates to a measuring process, the purpose of which is to increase the measuring accuracy of pulse oxymeters and comparable optical devices which are used in vivo to ascertain oxygen saturation of arterial blood.
According to the current prior art pulse oxymeters function on the basis that differing wavelengths blood attenuates light very differently depending upon the level of oxygenation. Pulse waves starting from the heart cause in the arterial blood vessel system a periodic fluctuation in the arterial blood content in the tissue. As a consequence, a periodic change in the light absorption (FIG. 1) can be registered between the light transmitter, whose radiation passes through the tissue, and the receivers, which are integrated in a pulse oxymetry sensor. The evaluation of the sensor signals is normally carried out at light wavelength of 660 and 940 nm by calculating the differential change of light absorption. It is possible to create a measured variable .OMEGA. (sometimes also referred to as R) which is obtained in the following manner or in a similar manner: ##EQU1##
The light intensities described in the formula represent the light intensities received in the receiver of the sensors used in pulse oxymetry. The measured variable .OMEGA. serves as a measurement for the oxygen saturation. The formation of a quotient in order to form the measured variable is intended to compensate any possible influences the haemoglobin content of the tissue, the pigmentation of the skin or the pilosity may have on the measurement of the oxygen saturation of arterial blood. (See also "Biomedizinische Technik" [Biomedical Technology] Volume 33, Supplementary volume 3, page 6 ff.:"Pulse oxymetrie: Stand und Entwicklung der Technik" [Pulse oxymtery: Status and developement of the technology"; Volume 35, Supplementary volume 1, page 38 ff. "Pulsoxymetrie" [Pulse oxymetry] by K Fortsner Institute for Biomedical Technology, Stuttgart). The influences of blood perfusion in the tissue, the pigmentation and pilosity are not taken into consideration in this measuring process.
When measuring oxygen saturation of arterial blood in the tissue in a range of 70 to 100% using light of wavelength 940 nm and 660 nm this also produces sufficiently accurate measured values. However, in order to measure lower oxygen saturation of arterial blood it is necessary to assume a strong influence on the measured variable .OMEGA. in particular caused by perfusion (i.e. blood content) (see: IEEE Photon Diffusion Analysis of the Effects of Multiple Scattering on Pulse Oximetry by Joseph M. Schmitt) and other optical parameters of tissue.
The dependency of the oxygen saturation of arterial blood SaO.sub.2 on the variable .OMEGA. and the perfusion p can be written as follows (see also FIG. 2): EQU SaO.sub.2 =f(.OMEGA.,p)
Similar influences can be caused by pigmentation and pilosity of the skin or scattering and inhomogeneous tissue.
The technical problem resides in the fact that oxygen saturation of arterial blood must be determined in vivo using the process of pulse oxymetry without the perfusion, scattering and inhomogeneity in the tissue or pigmentation and pilosity of the skin influencing the measured result. For this reason, it is necessary to locate from the number of possible calibration curves, those curves which render it possible to determine in the most precise manner the oxygen saturation of arterial blood.